The present invention relates generally to a pixel structure of an organic light -emitting diode (OLED) display device and its manufacturing method, and more particularly to a pixel structure of an active matrix full-color OLED display device and its manufacturing method.
Flat panel displays have become one of the most important electronic products such as notebook computers and pocket-TVs. Among the flat panel displays, organic electroluminescent (OEL) displays have the following advantages: light emitting, high luminous efficiency, wide viewing angle, fast response speed, high reliability, full color, low-voltage drive, low power consumption and simple fabrication process. Undoubtedly, this product has emerged as the display of choice in the market place. The structure and the manufacturing method of a conventional OEL display device of full color are summarized as follows:
(a) using precision photo-mask to get the pixel array of red, green, and blue (RGB) colors in a small molecules system,
(b) using color filter to get the pixel array of RGB colors on an OEL device of white light,
(c) using light converting layer to convert the original blue or purple light into other colors of light on an OEL device of blue or purple light,
(d) making medium stacking layers of various thickness to convert original wide-band light spectrum to RGB colors by the principles of the reflection and the interference of the light, and
(e) stacking devices of RGB colors on the same pixel element on a two-side transparent OEL device.
Although a conventional passive OEL display device has lower fabrication cost and simple process, its resolution is not high. It can only make a display device of small size and low resolution. However, an active drive, such as thin-film-transistor (TFT), OLED has characteristics of high resolution and low power consumption. Generally speaking, active drive technique is a major trend for high resolution and high display quality. As the needs for a display device of larger size and higher resolution, an active matrix OLED display device of full color becomes a major trend in the market.
U.S. Pat. No. 5,550,066 discloses a process for making a pixel structure of a thin-film-transistor organic electroluminescent display device. FIG. 1a and FIG. 1b show respectively a diagrammatic plan view and a cross-sectional view of this conventional TFT-OEL device. As shown in FIG. 1a, the pixel structure of a TFT-OEL device 100 comprises mainly two thin film transistors 101 and 102, a storage capacitor 103, and a light emitting OEL pad 104 arranged on a substrate. The TFT 101 is the switching transistor with the source bus 105 as the data line and the gate bus 106 as the gate line. The ground bus 107 is located above the gate bus 106 and below the storage capacitor 103. The source electrode of the TFT 101 is electrically connected to a source bus and the gate electrode comprises a portion of a gate bus. The OEL pad 104 is electrically connected to the drain of the TFT 102. The drain of the TFT 101 is electrically connected to the gate electrode of the TFT 102, which in turn is electrically connected to the storage capacitor 103. The TFT-OEL devices are typically pixel units that are formed into a flat panel display.
FIG. 1b is a cross-sectional view, illustrating the process of forming a pixel structure of this conventional TFT-OEL device. As shown in FIG. 1b, a polysilicon layer is deposited over a transparent and insulating substrate 111 and the polysilicon layer is patterned into a polysilicon island 118. Next, a first insulating gate layer 112 is deposited over the polysilicon island 118 and over the surface of the insulating substrate 111. A layer of silicon 114 is deposited over the gate insulator layer 112 and patterned by photolithography over the polysilicon island 118, such that, after ion implantation source and drain regions are formed in the polysilicon island 118. Ion implantation is conducted with N-type dopants. A gate bus 116 is applied and patterned on the insulating gate layer 112, and then a second insulating layer 113 is applied over the entire surface of the device. Contact holes are cut in the second insulating layer 113 and electrode materials are applied to form contacts with the thin-film-transistors. The electrode material attached to the source region of TFT 102 also forms the top electrode 122 of the storage capacitor 103. A source bus and a ground bus are also formed over the second insulating layer 113. In contact with the drain region of TFT 102 is the anode 136 for the OEL material. Then, an insulating passivation layer 124 is deposited over the surface of the device. The passivation layer 124 is etched leaving a tapered edge. The OEL layer 132 is then deposited over the passivation layer 124 and the anode layer 136. Finally, a cathode electrode layer 134 is deposited over the surface of the device.
There are still some difficulties to overcome for an organic electroluminescent display device of full-color in order to occupy the market of flat panel displays. For instance, it is lot easy to produce a display device of high resolution, high luminous efficiency and wide viewing angle. For example, the driver circuit becomes more complicated and more expensive to achieve the need of high resolution. The brightness and luminous efficiency are not good enough for an OEL device that employs small molecules as its organic material. Although an OEL device using polymer as its organic material has three colors of red, green and blue, its overall brightness and luminous efficiency are even worse than those of OEL device using small molecules as its organic material. Also, the materials employed in the OEL device are not compatible with photolithography. Therefore, it is necessary to develop a simple and efficient manufacturing method and structure for an organic light-emitting display device of full color.
The present invention has been made to overcome the above-mentioned drawbacks of a conventional organic light-emitting display device. The primary object of the invention is to provide a pixel structure of an active matrix OLED display device of full color. The pixel structure of the active matrix OLED display device of full color comprises mainly a color filter, two thin film transistors, a black matrix, a storage capacitor, and an OLED device structure constructed on a substrate. In this pixel structure of the OLED display device, the OLED device structure and the color filter are integrated in a thin-film-transistor array. This simplifies the fabricating process.
Another object of the invention is to provide a manufacturing method for the pixel structure of the full color and active-matrix OLED display device. The processing steps for the pixel structure are (a) the black matrix process, (b) the buffer layer process. (c) the island process (d) the gate process, (e) the interlayer process, (f) the metal layer process, (g) the passivation layer process, (h) the color filter process, (i) the transparent layer deposition process, and (j) the OLED deposition process.
According to the invention, the light path of the active matrix OLED display device of full color is as follows. Since the top metal layer is opaque, a light emitted from the OLED layer transmits downward, passes the transparent layer and the color filter, and finally traverses the transparent substrate. Therefore, a black matrix is added outside the color filter region and on the bottom surface of a thin film transistor in order to reduce the leakage of light and increase the contrast of the display device.
The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.